Walter Zinn

Walter Zinn
Born	(1906-12-10)December 10, 1906 Kitchener, Ontario
Died	February 14, 2000(2000-02-14) (aged 93) Clearwater, Florida
Citizenship	Canadian American
Alma mater	Queen’s University (BA 1927, MA 1930) Columbia University (Ph.D) (1934)
Awards	Atoms for Peace Award (1960) Enrico Fermi Award (1969) Elliott Cresson Medal (1970)
Scientific career
Fields	Nuclear physics
Institutions	University of Chicago Metallurgical Laboratory Manhattan Project
Thesis	Two-Crystal Study of the Structure and Width of K X-Ray Absorption Limits (1934)

Walter Henry Zinn (December 10, 1906 – February 14, 2000) was a nuclear physicist who was the first director of the Argonne National Laboratory from 1946 to 1956. He worked at the Manhattan Project's Metallurgical Laboratory during World War II, and is credited with initiating the world’s first self-sustaining nuclear chain reaction by withdrawing a control rod from Chicago Pile-1, the world’s first nuclear reactor on December 2, 1942 at the University of Chicago. At Argonne he designed and built several new reactors, including Experimental Breeder Reactor I, the first nuclear reactor to produce electric power, which went live on December 20, 1951.

Early life

Walter Henry Zinn was born in Kitchener, Ontario, on December 10, 1906, the son of John Zinn, who worked in a tire factory, and Maria Anna Stoskopf. He had an older brother, Albert, who also became a factory worker. Zinn entered Queen’s University, where he earned a Bachelor of Arts degree in mathematics in 1927, and a Master of Arts degree in 1930. He then entered Columbia University in 1930, where he studied physics, writing his Doctor of Philosophy thesis on "Two-crystal study of the structure and width of K X-ray absorption limits". This was subsequently published in the Physical Review.

To support himself, Zinn taught at Queen’s University from 1927 to 1928, and at Columbia from 1931 to 1932. He became an instructor at the City College of New York in 1932. While at Queen's he met Jennie A. (Jean) Smith, a fellow student. They were married in 1933, and had two sons, John Eric and Robert James. In 1938 he became a naturalised United States citizen.

Manhattan Project

In 1939, the Pupin Physics Laboratories at Columbia where Zinn worked were the center of intensive research into the properties of uranium and nuclear fission, which had recently been discovered by Lise Meitner, Otto Hahn and Fritz Strassmann. At Columbia, Zinn, Enrico Fermi, Herbert L. Anderson , John R. Dunning and Leo Szilard investigated whether uranium-238 fissioned with slow neutrons, as Fermi believed, or only the uranium-235 isotope, as Niels Bohr contended. Since uranium-235 was not available, Fermi chose to work with natural uranium. They were particularly interested in whether a nuclear chain reaction could be initiated. This would require more than one neutron to be emitted per fission, and by March 1939, they established that about two were.

At this point Zinn began working for Fermi, constructing experimental uranium latices. To slow neutrons down requires a neutron moderator. Water was Fermi's first choice, but it tended to absorb neutrons as well as slow them. In July, Szilard suggested using carbon, in the form of graphite. The critical radius of a spherical reactor was calculated to be:

$R_{crit}=-{\frac {\pi M}{\sqrt {k-1}}}$

In order for a self-sustaining nuclear chain reaction to occur, they needed k > 1. For a practical reactor configuration, it needed to be at least 3 or 4 percent more, but in August 1941 Zinn's initial experiments indicated a disappointing value of 0.87. Fermi pinned his hopes of a better result on an improved configuration, and purer uranium and graphite.

In early 1942, with the United States now embroiled World War II, Arthur Compton concentrated the Manhattan Project's various teams working on plutonium at the Metallurgical Laboratory at the University of Chicago. Zinn used athletes to build Fermi's increasingly large experimental configurations under the stands of the disused Stagg Field. In July 1942, Fermi measured a k = 1.007 from a uranium oxide lattice. This raised hopes that pure uranium would yield a suitable value of k.

By December 1942, Zinn and Anderson had the new configuration ready at Staff Field. Some 24 feet (7.3 m) long, 24 feet (7.3 m) wide and 19 feet (5.8 m) high, it contained 385.5 long tons (391.7 t) of graphite and 46.5 long tons (47.2 t) of uranium metal and uranium oxide. When the experiment was carried out on the afternoon of December 2 1942, the reactor, known as Chicago Pile-1, reached criticality without incident. Since the reactor had no radiation shield, it was run at a maximum power of only 200 W, enough to power a light bulb, and ran for only three months. It was shut down on February 28 1943, because the Army did not want to risk an accident near densely-populated downtown Chicago.

The Army leased a 1,000 acres (400 ha) of the Cook County Forest Preserves known as "Site A" to the Manhattan Project, and "the Country Club" to the 100 or so scientists, guards and others who worked there. Zinn was placed in charge of Site A, although Fermi still called the shots. Chicago Pile-1 was disassembled and rebuilt, this time with a radiation shield, at Site A. The reactor, now known as Chicago Pile-2, was operational again on March 20, 1943. Within a few months, Fermi began designing a new reactor, which became known as Chicago Pile-3. This was a very different type of reactor. It was much smaller, being only 6 feet (1.8 m) in diameter and 9 feet (2.7 m) high. It was power by 120 uranium metal rods, and moderated by 1,200 US gallons (4,500 L; 1,000 imp gal) of heavy water. Once again Zinn was in charge of construction, which commenced on New Year's Day in 1944. Chicago Pile-3 went critical on May 15, 1944, and commenced operation on June 23 at its full power of 300 KW. When Fermi departed for the Hanford Site, Zinn became the sole authority at Site A.

On September 29 1944, Zinn received an urgent call from Samuel Allison, the director of the Metallurgical Laboratory. The B Reactor at Hanford had shut down shortly after reaching full power, only to come back to life again some hours later. Norman Hilberry suspected a neutron poison was responsible. If so, it had a half life of around 9.7 hours. Xenon-135 had a half life close to that, but had not been detected in Argonne or by the X-10 Graphite Reactor in Oak Ridge, Tennessee. Zinn quickly brought Chicago Pile-3 up to full power, and within twelve hours, had made a series of measurements that confirmed the Hanford results.

Over the following months, some 175 technical personnel were transferred from the Metallurgical Laboratory to Hanford and Los Alamos. Zinn's Argonne Laboratory was reduced to a skeleton staff, but Compton would not countenance its closure.

Argonne National Laboratory

Zinn (standing) presses the button that closes down the Chicago Pile-3 unit for good.

On July 11, 1946, the Argonne laboratory officially became the Argonne National Laboratory, with Zinn as its first directory. At this time a new site, A new site about 5 miles (8.0 km)} away in DuPage County, Illinois, known as Site D, was designated as the laboratory's permanent home. The United States Atomic Energy Commission (AEC) replaced the Manhattan Project on January 1, 1947, and the following year Argonne was was became the AEC's main center for reactor development.

Zinn received multiple awards for his work, including a special commendation from the United States Atomic Energy Commission in 1956, the Atoms for Peace Award in 1960, the Enrico Fermi Award in 1969, and the Elliott Cresson Medal from The Franklin Institute in 1970. In 1955 he was elected as the first president of the American Nuclear Society (ANS).

Walter H. Zinn Award

Since 1976, the American Nuclear Society (ANS), Operations and Power Division, had annually presented the "Walter H. Zinn Award" to recognize an individual "for a notable and sustained contribution to the nuclear power industry that has not been widely recognized."

References

• Hewlett, Richard G.; Anderson, Oscar E. (1962). The New World, 1939–1946 (PDF). University Park: Pennsylvania State University Press. ISBN 0-520-07186-7. OCLC 637004643. Retrieved March 26 2013. {{cite book}}: Check date values in: |accessdate= (help); Invalid |ref=harv (help)
• Holl, Jack M.; Hewlett, Richard G.; Harris, Ruth R. (1997). Argonne National Laboratory, 1946–96. University of Illinois Press. ISBN 978-0-252-02341-5. {{cite book}}: Invalid |ref=harv (help)
• Weinberg, Alvin (1994). The First Nuclear Era: The Life and Times of a Technological Fixer. New York: AIP Press. ISBN 1-56396-358-2. {{cite book}}: Invalid |ref=harv (help)

Further reading

• "Walter Zinn 1907 - 2000". Canadian Nuclear Society. Retrieved February 14, 2011.